A nanoscale electric motor

Based on surface tension has been devised. Physicists at the University of California, Berkeley put two molten metal droplets—one of 30- and one of 90-nm radius—on the spine of a carbon nanotube. With a small DC voltage applied, atoms diffused along the CNT from the large drop to the small one, whose radius grew much faster than the large one’s shrank. In effect, potential energy was stored as surface tension in the smaller droplet. Eventually, the drops touched, and the higher internal pressure of the smaller one very rapidly drove the fluid back into the bigger one through the newly opened hydrodynamic channel. The cycle (shown in the photos) then repeated. The device is a “relaxation oscillator,” with a slow buildup and a rapid release of energy. Varying the voltage easily changes the operating frequency—a complete cycle took about 30 s at 1.3 V, about 6 s at 1.4 V, and about 1.2 s at 1.5 V. Attached to a mechanical linkage, the device could be used as a nanoelectro-mechanical actuator. (B. C. Regan et al., Appl. Phys. Lett. 86, 123119, 2005 http://dx.doi.org/10.1063/1.1887827 .)